According to Japanese Patent Application Laid-Open No. 2003-100702 (Patent Document 1), the etching device includes a substrate-holding rotation mechanism, an etchant supply mechanism, a film thickness measurement device, and control means. The substrate-holding rotation mechanism holds and rotates a substrate. The etchant supply mechanism supplies etchant on a film on the surface of the substrate that is being held and rotated by the substrate-holding rotation mechanism. The film thickness measurement device measures the film thickness of the film on the surface of the substrate that is held by the above substrate-holding rotation mechanism. The control means controls the supply of the etchant supplied by the above etchant supply mechanism based on the measurement result by the above film thickness measurement device while the above substrate is being held and rotated by the above substrate-holding rotation mechanism. The etching process is stopped when the measured film thickness reaches the target film thickness.
The above film thickness measurement device may include a light emitting unit, a light receiving unit, and a diffuser. The light emitting unit irradiates the target film to be measured with light. The light receiving unit receives reflected light from the target film to be measured. The diffuser which is disposed on the light receiving path leading from the target film to be measured to the above light receiving unit, diffuses the reflected light from the target film to be measured and emits toward the light receiving unit after homogenization. Accordingly, the above Patent Gazette claims that the reflected light from the target film to be measured is diffused and homogenized by the diffuser; therefore, the measurement of the film thickness is satisfactorily performed even while the target film to be measured is rotating.